- Medications to treat major depression act on a brain chemical called serotonin, which binds to receptors on brain cells. More research is needed on how serotonin receptors work in the brain, and imaging studies such as magnetic resonance imaging (MRI) can provide information on how these receptors function in the brains of individuals with depression and healthy volunteers. The experimental radioactive chemical [11C]CUMI has been designed to react with serotonin receptors, and researchers are interested in studying its effectiveness using positron emission tomography (PET) scanning to see how well it gets into the brain.